Up to now, a piezoelectric valve which is used to control the inflow and outflow of the fluid or the flow rate is disclosed in U.S. Pat. No. 5,628,411, for example. As shown in FIG. 13, this piezoelectric valve 1 is configured so that a diaphragm 3 whereon a piezoelectric element 2 is provided can be brought near or separated from a valve seat 5 of an output port 4. In this case, if the pressure of compressed air is introduced into a pressure chamber 6 from an input port 7 is P, a force Fp applied to the output port 4 is expressed by: EQU Fp=.pi./4.times.D.sup.2.times.P
(D: Diameter of opening of output port 4)
On the other hand, a recovering force Fs of the diaphragm 3 when the diaphragm 3 is displaced by an amount of displacement of x.sub.1 to close up the output port 4 is expressed by: EQU Fs=k.sub.i.times.x.sub.1
(k.sub.1 : Spring constant)
And, with this piezoelectric valve 1, by setting the pressure P of the compressed air so that Fp=Fs, a closed state is maintained.
On the other hand, when the output port 4 is to be opened, drive power is applied to the piezoelectric element 2 to change the amount of bending of the piezoelectric element 2 for separating the diaphragm 3 from the valve seat 5. This results in that the compressed air is allowed to flow from the pressure chamber 6 to the output port 4.
However, with the above prior art, there is a possibility of that, if the pressure P of the compressed air is varied, the force Fp applied to the diaphragm 3 is varied, as shown in FIG. 14, and when the recovering force Fs is larger than the force Fp, the diaphragm 3 is separated from the valve seat 5, resulting in the compressed air being allowed to leak from the output port 4.
The present invention has been provided to solve the above stated problem, being intended to offer a piezoelectric valve with which there is no possibility of the fluid leaking from the pressure chamber with a variation in the pressure of the supplied fluid.